The invention relates to purifying liquids and more particularly to further purifying ultra-pure liquids having a bulk resistivity in excess of one megohm-centimeter. The invention has particular applicability to the removal of particle contamination from ultra-pure water.
Ultra-pure water is generally produced by filtration of city water through a membrane, followed by a series of filtrations through a carbon bed and multiple ion exchange resin beds, with optional steps of ultraviolet sterilization and final membrane filtration through a "polishing" filter.
Ultra-pure water has only trace amounts of ionic, organic, and particle contamination.
By the use of further ion exchange resin beds, the amount of ionic contamination can be reduced to near the theoretical bulk resistivity of approximately 18.3 megohm-centimeters of absolutely pure water.
Particle contamination, including both biological particles (live and dead bacteria and bacterial fragments) and non-biological particles, is more difficult to control. These contaminants have a direct impact on product quality. Membrane filtration technology has some limitations in removing the trace amounts of ionic, organic, and inorganic particle contamination found in ultra-pure water. Membrane filtration systems include membranes having pores for the passage of the ultra-pure water therethrough. These pores are not consistent in size; rather, the pore sizes are distributed around some nominal size. Membrane filters will only remove particles larger than the actual pore size on which they happen to impinge. As a result, the particle concentration and distribution downstream of the membrane filter will reflect the membrane pore size and distribution.
Further, because of the minute size of the pores in a typical filter membrane, the filter produces a sizeable pressure drop on the order of five to twenty psid across the membrane. Often, this pressure drop results in an output stream pressure that is lower than desired for downstream processes.
In addition to drawbacks related to pore size, filter membrane material may deteriorate and shed particles from the physical impact of abrasive particles and chemical attack by the purified liquid. It is particularly difficult for filters to control organic or biological particles because in extended service, bacteria may form significant colonies on the downstream side of the membrane. This can occur by "grow through", fractionating into smaller endotoxins, or even by unnoticed remnant contamination left during membrane installation. Once established on the product side of the membrane, the bacteria colony will multiply and slough off more or less randomly into the product stream further contaminating the ultra-pure liquid. This is true even in the presence of sanitizing agents such as ozone. Moreover, even biological particles that remain on the upstream side of the membrane eventually disassociate or disintegrate, contributing smaller particles that pass through the membrane, as well as releasing organic contamination into the product stream. Finally, membrane filters can be cleaned only with cleaning solutions which are themselves likely to be contaminant-laden or by disassembly and back-flushing or replacement, both of which processes are liable to introduce new contaminants. Thus, it can be appreciated that membrane filtration may actually exacerbate the problem of particle contamination in ultra-pure water.
Ultra-pure water is used in a number of industries, such as the pharmaceutical and electronic industries. In the pharmaceutical industry, biological fragments (endotoxins) can contaminate fluids used for human injection such as water for injection (WFI). If contaminated fluids are injected into patients they may cause adverse physiological reactions. In the electronic device industry, the decreasing feature size of integrated circuits and their increasing overall complexity have made it important to control biological and non-biological particles for several reasons. Particles of one micrometer (10.sup.-6 meter in SI units) and larger may ruin the circuit element because they alter the intended image in the lithographic reproduction process. They can also damage the costly photomasks used in the process. Furthermore, submicron-size particles can reduce yields if they chemically alter either the silicon near the front surface or the composition of the films being deposited on the silicon wafer.
Standards for ultra-pure water are not entirely consistent. For example, USP standards for water for injection permit, inter alia, 50 colony forming units per ml of bacteria, 0.25 Eu/ml pyrogen, total solids of 10.0 mg/l, and a pH of 5.0 to 7.0. SEMI pure water guidelines for semiconductor processing label as acceptable 0.3 mg/l of total evaporative residue, 0.05 mg/l of total organic carbon, 1000 particulate counts per liter at 100.times. magnification, 6 live bacteria counts per 100 ml, and a resistivity of 17.9 megohm-cm.
Other examples of applications in which it would be desirable to have ultra-pure water which reliably contains fewer particulate contaminants than presently obtainable are micro-machining tools which rely on high-pressure water jets and the production of electrical capacitors using ultra-pure water as the dielectric medium.
These as well as other high technology industries will have an increasing reliance on the purest possible ultra-pure water in the future.